Point Action cameras, as they are referred to herein, go by many other names, including point of view cameras (see, e.g., pointofviewcameras.com), helmet cameras, action cams or action cameras, point of view shooter cams, video action cameras, and extreme sports cameras among others. Brand names include GoPro and ReplayXD. Conventional point action cameras typically have significant distortion, particularly at the outer several degrees of the field of view. In addition, astigmatism errors in conventional point action cameras can negatively impact the appearance of the video images that it captures. It is desired to have a point action camera or other compact digital camera that is capable of capturing a wide field of view, or a field of view that is greater than 90 degrees in either or both of the horizontal (x) and/or vertical (y) dimensions (or an arbitrary axis normal to the depth (z) dimension), and perhaps 135-150 degrees or more in the horizontal (x) dimension and/or perhaps 110-120 degrees or more in the vertical (y) dimension, and that is configured with built-in distortion and astigmatism correction.
Distortion in wide field of view cameras has been reduced with image processing software (see, e.g., U.S. Pat. Nos. 8,493,459 and 8,493,460, and US published patent applications nos. US20110216156 and US20110216157). It is desired however to alternatively provide a point action camera or other compact wide field of view digital camera, wherein the distortion that is typically inherent in wide field of view systems such as conventional point action cameras is compensated by an effective and efficient optical design.
Alex Ning describes a six lens design in U.S. Pat. No. 7,023,628 that has a ratio of total track length (TTL) to effective focal length (EFL), or TTL/EFL, that has a maximum value of 15 over which Ning states that the design would not have been considered compact. The Ning six lens design also has a minimum value of 8 under which Ning states that the design would not achieve the required fish eye field of view. U.S. Pat. No. 7,929,221 describes multiple optical assemblies that each include three aspheric surfaces on two lens elements and that each have a TTL/EFL ratio between 15 and 25. In an unrelated technical field, U.S. Pat. No. 7,675,694 nonetheless describes multiple optical assemblies that each include six aspheric surfaces on three lens elements. At U.S. Pat. No. 8,873,167, Ning describes an optical system that includes three lens elements in the first group and either having no aspheric lens elements or two aspheric lens elements, one in each of the two optical groups. In one example, Ning discloses a TTL/EPL ratio of 17.6, while in more compact examples TTL/EFL is described as being not less than 8.
It is also desired to have a compact camera design that captures reliably focused images. It is therefore desired to have an optical design that features a depth of focus that is greater than 20 microns.
It is recognized by the present inventors that it would be advantageous to have a design that is compact in having both a low TTL/EFL ratio and a low ratio of front element diameter to image diagonal and that achieves stable wide field of view image capture capability with tolerable, minimal, insubstantial, insignificant or drastically reduced distortion and astigmatism characteristics. Wang et al. have proposed at U.S. Pat. No. 9,019,629 an optical assembly for a mobile phone camera that exhibits a ratio of the sensor diagonal to the focal length between 1.27 and 1.55. The optical assembly of Wang et al. is not however configured for capturing images at a wide field of view.
It is also recognized by the present inventors that it would be advantageous to have a compact camera design that is carefully configured to suppress negative effects from stray light and achieve high dynamic range (HDR) imaging. It is therefore desired to have an optical design wherein near normal incidence surfaces are significantly avoided and at least 90% of return ghost foci are displaced at least 2 mm from the image sensor plane. In addition, it is desired to achieve a stray light irradiance ratio that is significantly below 1/1000, and even significantly below 1/10,000 in certain embodiments, and approximately at, or not greater than, or below 1/100,000 in certain embodiments, and a stray light power ratio that is significantly below 1/100 and approximately at, or not greater than, or below 1/1000 in certain embodiments.
As image sensor technology continues to improve, ever higher pixel densities are being achieved so that camera miniaturization need not imply low image quality in terms of resolution and contrast. It is therefore desired to have an optical assembly that has a high modulation transfer function or MTF at the Nyquist and half Nyquist frequencies. In this same context, it is desired to have an optical assembly capable of producing images without lateral chromatic aberrations larger than 2-3 pixels.